Vehicle mounted seismic sources, i.e. thumpers or vibrators, have long been known to those skilled in the art of seismic exploration. Examples include U.S. Pat. Nos. 3,288,244; 3,642,090; and 3,951,229. Vibrators are characterized by low energy output, but their frequency, which is usually quite low, can be controlled. Thumpers are more powerful, but produce a great deal of recoil and produce only a fixed, usually rather high, fundamental frequency. All of these devices are themselves extremely heavy and require heavy vehicles to carry and pre-bias them against the ground. This restricts their use in the field to readily accessible areas. Further, prior art devices generate a great deal of noise in addition to the seismic wave that produces returns of geological interest.
The prior art has taught the use of vacuum to lift heavy pressure plates used in conventional seismic sources. An example of this is shown in U.S. Pat. No. 3,983,947. The use of a partial vacuum to energize a seismic pulse generator for waterborne operation has been taught by the prior art. Examples include U.S. Pat. No. 3,349,867 and Canadian Pat. No. 608,169. Such devices cannot be used practically on land because they operate effectively only when coupled to the earth through water and a flexible diaphragm. They are also too weak to be widely useful. Nothing known to the applicant in the prior art teaches or suggests the use of a variable partial vacuum to tune the fundamental frequency of the seismic pulse. The large recoil characteristic of prior art seismic impulse generators requires that must be held to the ground by the weight of a large, heavy vehicle.
The closest prior art to the present invention is that taught by U.S. Pat. No. 3,189,121, issued to Van der Stoep on June 15, 1965.
Stoep teaches a seismic generator comprising a vacuum reservoir in external communication with a tube fitted with a movable piston. A base plate, not coupled to the tube, has a projection that enters the tube. This projection is covered by a fluid. The movable piston is propelled by atmospheric pressure to strike the fluid which, in turn, moves the base plate. The base plate may either set directly on the ground or may strike the pressurized fluid that is in contact with the ground by a flexible diaphragm. Stoep requires the use of a "heavy truck" to hold the source in contact with the ground. This is necessary because Stoep produces a great deal of recoil. Stoep even suggests the desirability of adding weight to the truck to "insure that the seismic pulse generator remains in contact with the surface of the earth" (Col. 2, lines 49-54, Stoep).
Stoep uses a controlled partial vacuum in its tube 10 and its connected vacuum reservoir 32 to do two things:
(1) vary the total energy generated in each seismic pulse (Stoep, Col. 2, lines 19-22); and
(2) reduce the tendency of the free piston to vibrate (Stoep, Col. 2, lines 11-13).
It must be noted that the amplitude and frequency of a seismic pulse are not the same. They should not be confused.
Amptitude is the total energy injected into the formation. Frequency is the rate at which this transfer of energy occurs.
In the present invention this rate of energy transfer is controlled by compression of residual air between the piston and the sliding base plate plug. Stoep cannot teach compression of this type because any residual air in tube 10 of Stoep would escape freely through port 35 into Stoep's relatively large vacuum reservoir 32.
Because Stoep teaches use of a relatively large vacuum reservoir, it cannot teach or suggest frequency control. Total energy, which is the variable Stoep controls by changing the degree of the evacuation of the tube and reservoir, is a function of the terminal velocity of the piston. As contrasted with Stoep, the present invention's difference of structure (no vacuum reservoir) and function (compression of residual air) leads to a synergistic difference in result (seismic wave frequency tuning).
Unlike Stoep, the present invention will work even when it is set against the ground at the end of the long arm of a "cherrypicker" truck. The present invention uses no oil and thus it can be operated at an angle to the vertical. The present invention can frequency match to the formation being studied.
Finally, Stoep's use of a vacuum reservoir suggests that the research of those skilled in the art of seismic wave production was proceeding in a direction that would have precluded frequency control as is taught by the present invention.